If you happen to receive a badly damaged lithograph (like a crease straight down the middle), you just contact Customer Support and they'll work you through the process of getting another. They always hold a few lithos back for this purpose and, in my experience, their customer care service is top-notch. Little bends in the corner don't matter much overall, as the image (should) be framed and will become unnoticeable.
I tend to go for custom frames, personally. But that's just me.

Everyone has their own style for framing these.
For the Kirkwall Gate, just get a frame that's a tad larger than the litho and fit a border (if the frame does not already come with one) around the litho.